A thin film transistor (hereinafter often abbreviated as TFT) has been widely used as a switching device for a display for a liquid crystal display device or the like. A representative TFT has a configuration in which a gate electrode, an insulator layer and a semiconductor layer are stacked in this sequence on a substrate, and has, on the semiconductor layer, a source electrode and a drain electrode being formed with a predetermined interval there between. The organic semiconductor layer constitutes a channel part, and an on-off operation is conducted by controlling electric current between the source electrode and the drain electrode by a voltage applied to the gate electrode.
Conventionally, this TFT was fabricated by using amorphous or polycrystalline silicon. However, a CVD (chemical vapor deposition) apparatus used for fabrication of a TFT using silicon is very expensive, and an increase in size of a display or the like using a TFT had a problem that the production cost increased significantly. Further, there was a problem that, since forming amorphous or polycrystalline silicon into a film requires significantly high temperatures, the type of a material which is usable as a substrate is limited, and hence, a lightweight plastic substrate or the like could not be used. There were also problems that plastic substrates or the like which were excellent in formativeness or flexibility could not be used.
In order to solve the problem, a TFT using an organic substance instead of amorphous or polycrystalline silicon has been proposed. As the film-forming method which is used when a TFT is fabricated by using an organic substance, a vacuum vapor deposition method, a wet coating method or the like are known. According to these methods, it is possible to realize an increase in size of a device while suppressing an increase in the production cost, and is also possible to allow the process temperature which is required at the time of film formation to be relatively low. Accordingly, in such an organic TFT, there are advantages that only small restrictions are imposed on the type of materials used for a substrate. Therefore, its practical use has been expected and research reports have been actively made.
As an organic semiconductor materials used for the active layer of the p-type field effect transistor (FET: field effect transistor), a polymer such as a conjugated polymer or thiophene, metal phthalocyanine compounds, and fused aromatic hydrocarbons such as pentacene or the like have been used singly or in the form of a mixture with other compounds. For the n-type FET materials, 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), 11,11,12,12-tetracyanonaphtho-2,6-quinodimethane (TCNNQD), 1,4,5,8-naphthalene tetracarboxylic diimide (NTCDI) and fluorinated phthalocyanine are known, for example.
As the device which uses semiconductors as in the case of an organic TFT, an organic electroluminescence (EL) device is known. In an organic EL device, in general, electric charges are forced to inject into a super thin film of 10 nm or less by applying a strong electric field of 105V/cm or more across the film thickness direction. In the case of an organic TFT, since electric charges are required to be flown at a high speed for a relatively long distance of several μm or more with an electric field of 105 V/cm or less, organic materials itself used in an organic TFT are required to have further conductivity than in an organic EL devices.
However, the above-mentioned organic semiconductor materials in conventional organic TFTs had small field effect mobility and a slow response speed, and hence were defective in high-speed response as a transistor. Moreover, the on-off ratio was also small.
In addition, the on-off ratio as referred to here means a value which is obtained by dividing a current flow between source and drain electrodes when a gate voltage is applied (ON) by a current flow between source and drain electrodes when a gate voltage is not applied (OFF). The on-current is normally means a current value at the time when the current flowing between source and drain electrodes is saturated (saturation current) after increasing the gate voltage.
Pentacene is known as a typical material for an organic TFT. An organic TFT which uses pentacene in an organic semiconductor layer is fabricated in Patent Documents 1 and 2. Since pentacene has a disadvantage that the stability in the atmosphere is low, although it shows a very high mobility immediately after the device fabrication, mobility will be lowered with the passage of time.
As a method to improve the stability in the atmosphere, a phenacene compound is used. For example, Non-Patent Document 1 proposes a picene ([5]phenacene) and Patent Document 3 proposes [5]-[11]phenacene. However, these compounds had a disadvantage that they had a low solubility in common organic solvents, and hence could not be applied to a wet coating process.
Patent Document 4 discloses an organic thin film transistor which is produced by a wet coating process and is provided with an organic semiconductor layer comprising a heterocyclic fused aromatic compound. Patent Document 4 states that this compound has excellent storage stability. However, in this document, the organic TFT for which the storage stability was evaluated were produced by the vacuum deposition method, and for organic TFTs which were produced by a wet coating process, evaluation of storage stability was not made.
As the method for improving the stability in the air, Patent Document 5 discloses a 5-membered fused ring structure in which thiophene rings are arranged on the both ends of a phenanthrene skeleton or a fused ring structure with 5 or more rings in which thiophene rings are arranged on one or both ends of a chrysene skeleton. However, there is neither a statement nor a suggestion on solubility.